The invention relates to a plastic suds tub for a washing machine, especially for a washing machine with a front opening for loading and unloading the laundry, which plastic suds tub has a cylindrical casing and an end face wall closing off the casing with a hollow-cylindrical, solid bearing mount for a bearing of a shaft of a laundry drum able to be rotated in the tub as well as means for stabilization.
The invention is based on a washing machine with a housing and a washing assembly suspended on springs within said housing, with the washing mechanism containing a cylindrical plastic suds tub, a washing drum supported to allow it to rotate therein, the axis of which has an essentially horizontal alignment, and a drive motor which drives the laundry drum directly or from outside via an intermediate belt pulley. The end face wall of the suds tub has a bearing mount to support the drum shaft.
Enormous rotational and bending forces arise during the operation of the washing machine, especially during spinning, which are generated by the rotational movement of the laundry drum and are transmitted to the suds tub. The loading on the suds tub increases with the size of the drum and the amount of the laundry loaded into it. The dynamics of the forces acting on the tub depend on the speed of rotation of the washing drum, the effect of the force on the tub increases especially when the drum is starting to rotate.
In front-loading washing machines the drum is supported on one side. In these washing machines known as front loaders the bending and rotational forces transmitted by the laundry drum must be taken up by the bearing mount in the end face wall of the tub and transmitted or distributed over the full surface of the latter. The requirements relating to mechanical durability are especially high with this washing machine type. Such a suds tub must be constructed to be strong enough to withstand all stresses over the long term, whereby consideration should be given for the construction of the tub to the fact that the mechanical stress is at its highest in the area of the bearing mount and reduces outwards from the bearing mount via over end face wall to the tub casing.
Assuming that the lifetime of a washing machine is more than 10 years, a tub must be designed so as to be able to fulfill all functional requirements over a this period of time, especially as regards its ability to withstand the mechanical stresses transmitted by the drum, as well as in relation to watertightness and corrosion resistance.
Another requirement for a modern mass product such as washing machines is that the individual components, such as the tub in the present case, must not only fulfill the functional requirements but must also be cost effective to manufacture as well as be able to be recycled without any problems once the washing machine has reached the end of its life.
So that the tub can fulfill all functional requirements over the long term, especially that of being sufficiently mechanically stable in relation to mechanical stresses, tubs for washing machines are known from the prior art in which numerous forms of means to stabilize the tub are also employed.
A particular tub construction is proposed in DE 199 52 991 A1. To optimize the tub in relation to the mechanical forces to be introduced and dissipated, the tub is constructed from an inner section made from a material resistant to washing liquor such as stainless steel or plastic and an outer section in the form of a supporting construction for accepting forces and for distributing the mass of the assembly.
Translating this type of construction into production is associated with a significant outlay in material and is also very demanding in terms of technology. Overall such a tub is not suited to economic mass production.
Tubs with a metallic bearing cross in their end face wall are in widespread use. An example of such a tub is disclosed in publication EP 1 528 136 A2. In this document it is proposed that a support contour, preferably made of cast iron, be arranged in the area of an end face wall, in the center of which a bearing seat is arranged for flying support of the drum by receiving a shaft support connected to the drum, with the support contour being at least embedded almost fully in the material of the end face surface. To securely dissipate forces and high temperatures from the support area the bearing contour possesses at least one or more arms running radially.
A known method of manufacturing these plastic tubs by injection molding is to first arrange the support cross in the injection machine mold and then undertake the injection itself. Insert-molding of the support cross makes it possible to seat it firmly in the end face wall.
The methods for manufacturing tubs with an insert-molded support cross are technologically demanding and expensive. In addition the injection molding methods also exhibit a number of problems and disadvantages which stem from the fact that materials are used that have very different properties.
Since the known tubs on the one hand involve a container made of plastic and on the other hand a support cross made from metal, their different material properties have a disadvantageous effect on the cooling-down process after injection molding such that, as a result of the different coefficients of expansion and the different thermal transfer capabilities, numerous stresses are produced after the metallic support cross is molded into the tub which are the result of the plastic shrinking when it cools down to room temperature.
The forces which act on the support cross in such cases are so great that the support cross can distort. A major disadvantage is that cracks can form in the plastic and the two different materials can came away from each other at the boundaries between them, which can lead after a longer period of operation to failure of the tub seals and to the support cross becoming loose in the end face wall. A further problem lies in the fact that the plastic inside the material cools down at a different rate from that at the boundary surface of the support cross. As a result micro gaps form between the plastic and the support cross, which worsens the connection between the materials and can lead to the formation of cracks.
A tub can be found in WO 2004/042133 A1 which has a metallic bearing shell which accommodates a member made of plastic during injection molding which is more solid and of better quality than the plastic forming the container. The metallic bearing shell and the plastic member form a unit onto which the plastic tub is injection molded during a further process step.
This method is designed to counter the formation of cracks in the area of the bearing mount described above. The proposed construction offers the further advantage that the additional member made from more solid plastic gives the tub a higher mechanical stability in the area of the bearing mount. The disadvantage of the construction lies in the stabilizing effect of the additional member being restricted to the area of the bearing mount. The solid plastic member has no effect, or only an insignificant effect on the stability of other areas of the tub, especially the end face wall of the tub containing the bearing mount.
As an alternative to the plastic tubs described with the support cross integrated into the end face wall, types have been developed with stabilizing means consisting of plastic. The unification of the technology that this makes possible produces not insignificant rationalization potential, with the technical and financial effort involved in manufacturing the tub being significantly reduced.
For example a tub is described in DE 20 2004 012 221 U1 of which the rear end face wall is provided with a plurality of straight rigid ribs which are arranged at the same angular spacing from each other and starting from the through bearing mount, which is arranged in the middle of the rear end face wall, run radially outwards to the outer edge of the rear end face wall.
Tubs are also known which in the area of the interface wall, in addition to the radial reinforcement ribs, feature serpentine, star, oval, circular or spherically molded-in stress relief profiles. The profiles are designed so as to be well suited to forces running in a number of directions.
Such tubs exhibit a greater rigidity compared to the example given above. The disadvantage of these tubs is that accumulations of material occur in the areas where the different stress relief profiles cross over each other, which as a result of the different temperature gradients in some areas, causes the material to shrink differently after injection molding. The result is material stresses within the end face wall with the resulting danger that cracks can form in the material and after a longer period of operation to the seals of the tub failing.
The known constructions of tubs with stabilization means made of plastic described are not able to be used or only able to be used to a restricted extent for larger load volumes and very high spin speeds.
The rigidity of a tub constructed in this manner can be increased up to a certain level by designing the material strength of the end face wall and/or of the stabilization means formed into it to be higher or stronger or by using more solid plastics. Since more solid plastics essentially make the tub more expensive and strengthening the end face wall and the stress relief profiles leads to accumulations of material, relatively narrow limits are imposed on both options for strengthening the end face wall. With modern machines in particular, with spin speeds of the rotating drum of over 1500 rpm and a load of more than 8 kg, the known constructions of tub with stabilization means made exclusively of plastic are not able to be used.